The CdS and CdS/SiO2 photocatalyzed addition of 2,5-dihydrofuran (2,5-DHF) to azobenzene in methanol solution affords 3-(2,3-dihydrofuryl)hydrazobenzene (1) as the major and hydrazobenzene (2) as the side product. According to 13C NMR and adsorption data, 2,5-DHF is adsorbed parallel to the surface through hydrogen bonding with [OH/SH] groups; adsorption constants of 18 and 30 L mol-1 are obtained for CdS and CdS/SiO2, respectively. This enhanced effect of the silica support favors formation of the addition product 1. Different from that, azobenzene exhibits adsorption constants identical within experimental error (11126 and 1059 L mol-1) for both photocatalysts and adsorbs most likely at Brønsted acid centers. These results are corroborated by corresponding adsorption studies with SiO2. The much better adsorption of 2,5-DHF onto CdS when performed in aqueous suspension is rationalized by comparison of adsorbent and adsorptive polarities, as determined by Reichardt's dye. The absence of any addition products of methanol to a mutual dihydrofuryl radical cation suggests that oxidation by the reactive valence band hole occurs via dissociative electron transfer affording a proton and the dihydrofuryl radical. The latter undergoes C−N coupling with PhN−N(H)Ph, formed in a proton-coupled reduction by the reactive electron, to afford 1. Formations of 1 and 2 exhibit the same Arrhenius activation energies of 11 kJ mol-1. The rates of both reactions are decreased upon increasing the solvent viscosity by increasing pressure or by using various alcohols. From the average activation volume of 17 cm3mol-1, obtained for 1 and 2 at pressures up to 120 MPa, it is postulated that radical diffusion in the solvent−solute surface multilayer rather than C−N bond formation is the rate-determining step.

Time-resolved infrared and time-resolved optical spectroscopy have been used to probe further into the spectra and dynamics of reactive intermediates I and I ind generated by flash photodecarbonylation of the respective acetyl complexes CpFe(CO)2(C(O)CH3) (A) and IndFe(CO)2(C(O)CH3) (A ind ) (Cp = η5-C5H5; Ind = η5-C9H7). The competitive reaction dynamics of CH3 migration to the metal, trapping by CO, and trapping by other ligands such as P(OCH3)3 were determined for these intermediates in various solvents. Hydrostatic pressure effects on the competitive photoreaction pathways for both A and A ind in hexane solutions were also examined, and it was found that the photosubstitution pathway has a significantly more negative activation volume than does the photoinduced methyl migration (although a less negative value was found than that previously reported from these laboratories). The indenyl intermediate I ind is about 5-fold more reactive toward methyl migration and toward trapping by various ligands than is the cyclopentadienyl analogue I, but these differences appear to be too small to support a ring-slip mechanism for either type of reaction. The overall picture points to solvento species, e.g., CpFe(CO)(Sol)(C(O)CH3), as the most likely form of the intermediates for all solvent systems studied at ambient temperature, with the possible exception of the solutions in perfluoro(methylcyclohexane). The possible relevance of these species to mechanisms for migratory insertion of CO into the metal−alkyl bonds of the methyl complexes CpFe(CO)2CH3 (M) and IndFe(CO)2CH3 (M ind ) is discussed.

The kinetics of carbon monoxide binding and dissociation have been studied for a series of lacunar iron-(II) cyclidene complexes to elucidate the dependence of dynamic parameters on the various structural features of these versatile compounds. Ligand substituents have large effects on the binding and dissociative rate constants, and remarkably, four distinctly different steric effects have been observed. (1) Changing cavity size alters the rate of CO binding by as much as 4 orders of magnitude, presumably by constraining access to the metal ion. (2) Decreases in cavity size also can increase the rate of CO dissociation by a factor of 10 or so. (3) Placing bulky groups in the path CO must follow to enter the cavity decreases the rate of binding because of steric effect (1), but these same obstructions may also decrease the rate of dissociation by blocking the escape path and, possibly, fostering geminate recombination. (4) Proximal ligand strain both decreases the rate of binding and increases the rate of CO dissociation. In contrast, changes in the iron(III)/(II) redox potential, which accompany ligand substitutions, were found to have only a small impact on CO binding kinetics. The effects on the rate constants of the basicity of the axial base and of solvent polarity were also investigated.

The activation volume for the reaction of CO2 with ONOO- has been determined to be −22 cm3 mol-1 using the high-pressure pulse radiolysis technique. The significant negative volume of activation provides conclusive evidence for a short-lived cyclo-adduct formation between ONOO- and CO2.